1. Field of the Invention
The present invention relates to a long-sized tubular, usually grounding, container unit for a gas-insulated electrical device such as a gas-insulated bus and a gas-insulated switch gear device, and a laser welding device for manufacturing the same.
2. Description of the Related Art
A gas-insulated electrical device which is known, such as a gas-insulated bus and a gas-insulated switch gear device, generally comprises a grounding container filled with an insulating gas, such as SF.sub.6 gas, and a conductor supplied with a predetermined potential and disposed in the grounding container while being electrically insulated therefrom.
An example of a conventional grounding container for a gas-insulated electrical device is disclosed in JP-A-59-189092 (1984), wherein the grounding container is constituted by a plurality of tubular barrel units each having flanges at both axial ends, and the respective tubular barrel units are connected by bolts and nuts through the corresponding flanges to complete the grounding container. Further, JP-A-59-189092 (1984) discloses to connect a tubular barrel portion and two flange portions for the tubular barrel unit by a laser welding. Tubular barrel units having a good dimensional accuracy are thereby manufactured in a short time through a single butt welding with only a small thermal deformation.
Another conventional grounding container is constituted by connecting a plurality of tubular barrel units with no flanges at the ends through arc welding.
The axial length of the respective tubular barrel units is relatively short because of a practical limitation imposed by the mode of transporting the units. Therefore, many tubular barrel units are usually needed to complete a grounding container having a required axial length.
Further, the production cost of the former grounding container is high, because the respective tubular barrel units are provided with flanges at both ends which have to be provided with a groove for receiving an O-ring for maintaining airtightness at the flange coupling.
The production cost of the latter grounding container is lower than the former one; however, when an accident such as grounding occurs in the grounding container which is operating as a part of a gas-insulated electrical device, it is necessary to cut and disassemble the grounding container at the vicinity of the accident point in order to exchange parts therearound, which takes a long time.
Further, the laser welding disclosed in JP-A-59-189092 (1984) is performed by rotating the tubular barrel portion and the flange portion with respect to a laser welding device. Therefore the laser welding device cannot be applied to a container which is fixed and cannot be rotated, such as when an additional long-sized tubular container is to be connected by laser welding to a previously installed container.
Further, JP-A-3-128183 (1991) discloses a laser working device wherein an inner cylindrical tube inserted in an outer cylindrical tube is welded to the outer cylindrical tube by a laser beam moving around the inner circumference thereof.
Still further, JP-A-59-191584 (1984) discloses a tubular body working device welded by a laser beam moving around the outer circumference of a tubular work body.
The laser working device disclosed in JP-A-3-128183 (1991) employs an optical fiber to transmit the laser beam, and is suitable for working a thin tubular body having a small diameter. Therefore, if the laser working device is applied for a long-sized tubular container having a large diameter, rigidity of a supporting member supporting a working head, including a condenser lens and a reflecting mirror, has to be redesigned, and many man-hours and a large space for setting the work head are required before and after the welding operation.
When the laser beam disclosed in JP-A-59-191584 (1984) is applied to welding in all attitudes, control of the laser beam focusing position is necessitated because the distance between the laser oscillator and a work point varies during the working operation. The control system complicates the mechanism of the device.
Still further, as disclosed, for example, in JP-A-3-9834 (1991) a laser working device which makes use of an optical fiber is known. However, such a conventional device cannot be used immediately such as for welding a container by making use of a large output laser for a long time because the device suffers from excessive energy losses in a bent optical fiber.
The laser-welded joint portions of a grounding tubular container used for a gas-insulated electrical device have to withstand a predetermined pressure. Therefore, a pressure test or an airtightness test of the welded joint portions is obligatory. However, no specific measures which facilitate the pressure test are provided for the conventional long-sized tubular container.